1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, in particular, to an alkali metal electrochemical cell or a lithium ion rechargeable electrochemical cell capable of discharge at elevated pressures and temperatures.
2. Prior Art
It is known that the useful operating temperature range of a lithium electrochemical cell can be extended beyond 180.54xc2x0 C., the melting point of lithium, and up to approximately 200xc2x0 C. The operating temperature is increased, in part, by providing the anode as a lithium alloy, such as a lithium-magnesium alloy. Although functional, conventional lithium cells manufactured for such high temperature applications exhibit serious deficiencies. In particular, the energy density of a prior art lithium cell is decreased due to the use of a ferrule included in the header assembly. The ferrule is required to maintain electrical isolation of the terminal lead within the header assembly and to prevent the cell from leaking.
According to the present invention, it has been discovered that the energy density of an electrochemical cell, either of a primary or a secondary chemistry, intended for use in elevated pressure and temperature applications can be significantly improved through the use of a novel header assembly which eliminates the use of a ferrule.
The present invention describes an improved header assembly. The present header assembly retains hermeticity when the cell is discharged under a high rate at an elevated temperature while simultaneously increasing the cell""s energy density by allowing for the inclusion of additional electrochemically active components. During discharge at temperatures up to about 200xc2x0 C., lithium cells of either a primary or a secondary chemistry produce gaseous by-products which increase the pressure within the cell. Therefore, the header assembly including the glass-to-metal seal must be capable of withstanding this increased pressure while retaining hermeticity under all use conditions. If hermeticity is not retained, the compromised cell is inoperable and may vent or explode.
As discussed earlier, prior art lithium cells typically contain a ferrule which supports the glass-to-metal seal. The ferrule serves to minimize deflection of the header assembly and to reduce internal stresses and forces placed upon the glass-to-metal seal. Although prior art ferrule constructions allow a lithium cell to remain operational at temperatures up to about 200xc2x0 C., there are several drawbacks to them. First, conventional ferrules typically take the form of a ring welded to the header assembly. Therefore, simply having a ferrule increases the cost of the header assembly and the resulting cell. Secondly, the ferrule ring is two to six times the thickness of the lid and projects downward into the cell, reducing the height available for the electrochemically active materials. This leads to a reduction in the energy density of the cell.
A novel header assembly according to the present invention eliminates the use of a ferrule. The lid of the header assembly is modified to include a concentric groove of a fixed width and depth. The groove is radially located a fixed distance from the edge of the glass-to-metal seal. The thickness of the lid and its location, width and depth depend upon the specific electrolyte used to activate the cell, the cell size and the intended discharge rate and temperature of the cell, among other parameters. The present header assembly is particularly useful in high temperature lithium cells and results in a significant increase in the cell""s energy density through the inclusion of additional electrochemically active components. When discharged at temperatures up to about 200xc2x0 C., the header retains hermeticity.